It's really hard to pick and I have to say that I change my mind regularly because I listen to them so much and then I'll listen to one I haven't heard in a while and it all changes. I do love both Tarzan and Lion King, but like the soundtracks to the Broadway shows even more (extra content). Same with Little Mermaid and Mary Poppins, but not as much. Princess and the Frog is definitely up there in the top 5.
I've always felt one of the most underated was Emporer's New Groove. I was so looking forward to this movie after devouring the soundtrack, only to find most of the songs were not used. How can you cut Ertha Kitt's only song??
Never listened to the complete soundtracks from any of the sequels, but I do like some of those songs as well.